关于各种of which 和of them的句子！

你在瞎折腾！研究半天，总结出什么乱七八糟的东西？
which, 与 that-, who-/whom, 都属于“关系代词”，关系代词是与复合句有关的，是用来引导定语从句的。
them,与 he/she, it, you I me, us等属于同一的概念范畴，是普通的人称代词，代替前面提到的人或物，避免重复提及前面已经提过的相同的名词。
所以，判断的关键，是要从句子的结构入手，看这个句子：
1、简单句1。简单句2。用 them.
I teach 40 students. Twenty of them are girls.
2. 并列句：简单句1 and 简单句2。用them.
I teach 40 students and twenty of them are girls.
3. 复合句：主句+ 关系代词(which) + 定语从句。
I teach 40 students (,) twenty of which/ whom are girls.
明白了？
来看你的句子：
1、2句和我的3句是一样的，主句+ 非限制性定语从句，用关系代词，which-
但是一旦改变你的句子结构，表达就会改变：
I was given three books on cooking. I really enjoyed the first ( of them ) .
两个独立的简单句，用人称代词，them.
I was given three books on cooking and I really enjoyed the first （of them).
用 and 连接的一个并列句，用人称代词：them.

关系代词who、which作主语时，谓语动词的人称、单、复数与先行词必须一致，当先行词为单数时用which is，先行词为复数，则用which are。
如：
Thomas is one of the boys who are fond of sports.
I know a boy who is good at surfing the Internet.
Adults don't like children who tell lies.